Bright field microscopy, where a digital video camera observes particles from a direction perpendicular to an illuminated background surface, is used to detect the size and shape of particles with diameters smaller than a few millimeters down to a few microns. When examining particles in dry particulate samples, prior art teaches using a sample feed where particles are either falling or are moved by compressed air to scatter the particles, such as what is presented in Annex C of ISO application 13322-2:2006, titled Particle size analysis—Image analysis methods—Part 2: Dynamic image analysis methods, incorporated herein by reference. In that Annex C, several examples of sample feeds for bright field microscopy applications are provided: a sheath flow cell, a free-falling system, and the measurement of particles on a moving substrate or at a conveyor discharge point.
A sheath flow cell, which is the most commonly used method, involves forming a sheath with high pressure air that separates particles so that they do not obstruct one another in front of the digital camera. When measuring particles on a moving substrate, there is a particle dispenser moving over the substrate (e.g. a glass plate), while the camera scans the entire area of the substrate, which is illuminated from underneath. Free falling systems or measurement of particles at a conveyor discharge point are not popular methods, due to the lack of precision and control in how the particles are separated, and therefore are rarely deployed in practice. All the above methods present the fundamental problem of particle movement during exposure, which can create fuzziness in the image, depending on the speed at which the particles are moving.
As a non-limiting example, a digital camera may have 5-micron by 5-micron pixels and 2× zoom that allows for the visualization of sub-millimeter particles on a 2000-pixel by 1000-sensor, which corresponds to an optical system with a calibration constant of 2.5 microns per pixel linear size. If a short exposure of 100 microseconds is used, the speed of moving a distance of one pixel is equivalent to 2.5 cm/sec. This is a very low speed when compared to free falling speed in Earth's gravitational field, which results in about 2 m/sec speed after just 20 cm is traveled by particles. Moreover, if high pressure air is being used to separate the particles to form a sheath, the obtained speeds are even higher. In some commercial apparatus, the speed can be up to 50 m/sec, thus requiring very short exposure times to not result in fuzzy images, and which thus requires very high intensity illumination to capture clear images. The intensity of light required would have to be high power laser light sources that are technically difficult.
What is needed, therefore, is a particle analyzer that overcomes these shortcomings.